The continually increasing combustion of fossil fuel, such as coal, natural gas and oil, during the last centuries has resulted in an increase in the concentration of CO2 in the atmosphere. The increasing concentration of CO2 has caused concern due to the greenhouse effect caused by CO2. The greenhouse effect is suspected already to have caused at least some of the changes in the climate that have been seen during the last decades, and is according to simulation models suspected to cause even more and potentially dramatic changes in the climate of planet earth.
This has caused a call for action from scientists, environmentalists and politicians throughout the world, to stabilize or even reduce the discharge of CO2 from combustion of fossil fuel to the atmosphere. This may be achieved by capturing and safe depositing of CO2 from the exhaust gas from thermal power plants and other plants where fossil fuel is combusted.
The captured CO2 may be injected in sub terrain formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for deposition. Tests indicate that CO2 remains in the sub terrain formation for thousands of years and is not released into the atmosphere.
The capture of CO2 resulting from combustion of biofuel is also considered as a method of reducing CO2 emissions to the atmosphere. Biofuel is considered a renewable fuel and combustion of biofuel has no effect upon CO2 concentrations in the atmosphere. Capturing CO2 from the exhaust from a biofuel plant will reduce the CO2 concentrations in the atmosphere.
Capturing of CO2 from a gas by means of absorption is well known and has been used for decades, e.g. for removal of CO2 (and other acid gases) from produced natural gas at gas fields. The absorbents used or suggested in the prior art have been different aqueous alkaline solutions, such as potassium carbonate, see e.g. U.S. Pat. No. 5,528,811, and different amines, see e.g. U.S. Pat. No. 4,112,051, U.S. Pat. No. 4,397,660 and U.S. Pat. No. 5,061,465. Additionally, an aqueous ammonia solution has been suggested as an absorbent for CO2. Separation of CO2 from exhaust gas from thermal power plants by means of an amine solution, is known e.g. from U.S. Pat. No. 4,942,734.
Common for these CO2 capturing solution is that the gas mixture to be separated is introduced countercurrent to the aqueous adsorbent in an absorber column. The gas leaving the absorber column is CO2 depleted (or acid gas depleted), whereas the CO2 (or other acid gas) leaves the absorber column together with the absorbent. The absorbent is regenerated in the regenerator column and returned to the absorber column. Amine is regenerated by stripping the amine solution with steam in the regeneration column. The steam is generated in the reboiler at the base of the column.
As illustrated above CO2 as such is well known in the art. However, there is a need for several improvements in the CO2 capturing process to make CO2 free or low CO2 emission thermal power plants economically profitable.
Capturing of CO2 is carried out at the expense of the efficiency of a thermoelectric power plant utilizing fossil fuel, so that the output of electrical power and/or medium temperature heat from a thermoelectric power plant is reduced. The reduced efficiency compared with a traditional plant makes these facilities less profitable. Improvements in the efficiency, i.e. reducing the energy cost in the CO2 capturing process, are therefore sought.
The currently preferred absorbents are aqueous solutions of different amines. The commonly used amines are alkanol amines, such as e.g., diethanol amine, mono methyl ethanolamine, aminoethyl ethanolamine, 2-(Methylamino)ethanol, MDEA as well as other amines known by skilled man in the art. The absorption of CO2 to the amine absorbents is a reversible, exothermic reaction. Accordingly, heat has to be supplied to the regenerator column to reverse the absorption and release the CO2. NO321817 (corresponding to WO2005/045316) describes a method and plant for capturing CO2 from a thermal power plant, where the flue gas from the power plant is cooled, compressed and used as an oxygen containing gas, optionally with added air or oxygen, in a pressurized combustion chamber for combustion of natural gas. The combustion in the pressurized combustion chamber is substantially stoichiometric, resulting in an exhaust gas having a substantially reduced oxygen concentration and a higher concentration of CO2. The exhaust gas from the pressurized combustion chamber is cooled and kept at the elevated pressure to allow for a more efficient absorption of CO2 in an absorption device for separation of the exhaust gas to a CO2 depleted fraction that is released into the surroundings, and CO2 to be deposited.
WO2006/043820 describes a method for capturing CO2 wherein exhaust gas from a first gas turbine is cooled and fed as oxygen containing gas into a second gas turbine. The exhaust gas from the second gas turbine is again cooled and separated into a CO2 depleted fraction that is released into the surroundings, and CO2 to be deposited. The two step combustion increases the concentration of CO2 in the exhaust gas from the second gas turbine to be separated and thus increases the efficiency of the CO2 absorption in the separation part of the plant.
Both NO 321817 and WO 2006/043820 are, however, dependent on additional supply of fossil fuel for the enrichment of CO2 in the second combustion.
In Tel-Tek report No. 2106040-1 of June 2006 (Marit Larsen, Tel-Tek Dept. GassTEK, Porsgrunn, Norway) it has also been suggested to use wood chippings as a source of heat energy for the CO2 absorption/desorption process. Exhaust gas from a gas power plant may be cooled by drying of wood chippings. The dry wood chippings may then be combusted in a steam boiler to produce to produce steam for the regeneration of the CO2 absorbent. The combustion gas from the steam boiler may be released into the atmosphere, as the CO2 is from an organic source, or be mixed with the exhaust from the gas power plant and introduced into the CO2 capturing plant. Capturing CO2 from combustion of wood chippings makes it possible to capture more CO2 than the CO2 generated from natural gas, and thus makes it possible to increase the efficiency and thus profitability of the plant. No secondary combustion using the exhaust from the gas power plant is, however, suggested or indicated as a possibility.
It is an objective of the present invention to provide a CO2 capturing solution for an existing or new fossil fuel fired thermal power plant that is more energy efficient than the solution according to the prior art.
It is also an objective of the present invention to provide a CO2 capturing for a fossil fuel fired thermal power plant resulting in low or no net entry of CO2 of fossil origin into the atmosphere.